In recent years, a wireless mesh network (also known as a wireless mesh network system), which organically combines a plurality of wireless devices by a wireless mesh, has been drawing attention (see Non-Patent Document 1). The wireless mesh network functions effectively by having a plurality of wireless devices (wireless stations) scattered around such that it can concentrate data at a point and make it possible to have communications from a particular wireless device to another particular wireless device even when direct one-to-one communications therebetween are not possible.
In case the wireless device is a mobile terminal, the wireless mesh network is usually formed as an ad-hoc network. In the ad-hoc network, data is transmitted from a transmitting node to a receiving node via a plurality of wireless devices serving as wireless terminals (nodes). This technology is also referred to as a multi-hop, and a routing (selective control of paths) method has been suggested as a typical example thereof (see Patent Documents 1 and 2).
In the routing method, a communications path is specified by providing a routing table in each of the wireless devices. As an example, there has been suggested an ad-hoc on-demand distance vector (AODV) wherein each node knows only the next relay station. Further, as for a communications protocol, ZIGBEE (registered trademark) serving as the standard for Low Rate Wireless Personal Networks, LR-WPANs (IEEE 802.15.4), is known as a typical example (see Non-Patent Document 2).
As another multi-hop technology, a flooding method is known (see Non-Patent Document 1). In the flooding method, a wireless device receives data from another wireless device and continuously and unconditionally transmits data on its own accord and, by repeating reception and transmission like this, the data is finally forwarded to a destination wireless device.
However, since the routing method is based on ‘distributed processing’ in which there exists no station for centrally controlling each wireless device, information about other wireless devices capable of local communications is required. Also, such information needs to be stored in a memory of each wireless device, and the capacity of the memory ranges from 400 B (byte) to 3 KB (kilobyte).
This size of the memory capacity has been a heavy burden on the premise that an embedded small central processing unit (CPU) is used for a communications controller of a wireless device. Due to this, making the wireless device smaller has been difficult. Moreover, routing processes are complicated and thus a CPU for executing the routing processes needs to have a high processing capability. Therefore, the prices of CPUs are high and power consumption is high.
Further, in the routing method, there may be needed changing a communications path depending on a change in the propagation state of radio waves regardless of whether each wireless device is a mobile or in a fixed position. Accordingly, the processing load of the CPU has become burdensome. Especially, in case a wireless device serving as a communications terminal is a mobile terminal, proper communications cannot be achieved unless the communications path is changed every moment. Therefore, path search needs to be made at regular intervals, which further increases the processing load of the CPU.
Additionally, for example, in the event of a failure of a wireless device included in the communications path, if the propagation state of the communications path is rapidly changed, the update processing speed of the routing tables cannot catch up with this change, and thus may lead to failing to establish a communications path.
On the other hand, in the flooding method, each wireless device is characterized in that it has no information about a communications path. A wireless device receives data from another wireless device and continuously and unconditionally transmits the data on its own accord, and, by repeating this, the data finally is transmitted to a destination wireless device. Due to this, multiple wireless devices are simultaneously brought into a transmitting state in disorder.
As a result, radio traffic increases and radio waves are overcrowded, thus failing to maintain the quality of the communications path. Moreover, each wireless device is in a transmitting state over a long period of time so that power consumption of each wireless device is large. Especially, if the wireless device operates with a battery, it cannot last for a long time.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2005-64721
Patent Document 2: Japanese Patent Laid-Open Publication No. 2008-66861
Non-Patent Document 1: “Sensor Network Technology”, Shigeru Ando et. al., first issue, first edition, published in May 20, 2005, Tokyo Denki University Press
Non-Patent Document 2: “ZigBee”, Li Zheng, first issue, first edition, published in Feb. 22, 2006, RIC TELECOM Co., Ltd.